SiC has three major crystal types: 3C, 4H, 6H. Since the crystal growth of SiC is technically difficult compared to that of silicon, 6H—SiC wafer was first communalized in the 1980s as a substrate for growth of GaN, which is used for blue LEDs. In the 1990s, it was demonstrated that 4H—SiC is suited for power electronic devices and the commercialization of 4H—SiC wafer has started. Today, 4H—SiC wafer is much more produced than 6H—SiC. 3C—SiC wafer is not yet commercialized.
It has been known that SiC shows piezoresistive effect. SiC has long been viewed as a potentially useful semiconductor for high temperature transducers. Several high temperature “pressure” sensors up to ˜650° C. have been reported. Most of them used 6H—SiC. It is most likely because of the SiC wafer history. The following publications mentioned 4H—SiC high temperature pressure sensors:                R. S. Okojie, D. Lukco, C. Blaha, V. Nguyen, and E. Savrun, “Zero offset drift suppression in SiC pressure sensors at 600° C.,” in Proc. TREE Sensors conference 2010, pp. 2269-274, 2010.        R. S. Okojie, V. Nguyen, E. Savrun, and D. Lukco, “Improved reliability of sic pressure sensors for long term high temperature applications,” in Proc. Transducers '11 Beijing China, pp. 2878, 2011.        
U.S. Pat. No. 7,516,668 describes a high temperature transducer fabricated from silicon carbide. It is fabricated by first epitaxially growing a layer of highly N-doped 3C silicon carbide on a first silicon wafer or substrate. A second wafer of silicon carbide, selected to be a carrier wafer, is etched preferentially to produce the deflecting diaphragms The 3C material is patterned to provide a series of individual piezoresistors which may be interconnected to form a Wheatstone bridge.
US 2004/0173027 describes a semiconductor pressure sensor which includes a semiconductor substrate having a diaphragm for receiving pressure and a bridge circuit for detecting a distortion of the diaphragm corresponding to the pressure. The bridge circuit includes a pair of first gauge resistors disposed on the center of the diaphragm and a pair of second gauge resistors disposed on a periphery of the diaphragm
U.S. Pat. No. 5,184,515 describes a transducer having a plurality of sensing elements disposed in a single diaphragm wherein each of the sensing elements is spaced from every other of the sensing elements a predetermined distance so as to control interference among the sensing elements. Each of the sensing elements preferably comprises a plurality of piezoresistors each of which are coupled in a Wheatstone bridge configuration.
US 2008/0022779 discloses a pressure sensor comprising a membrane on which one first measurement element and one second measurement element for detecting a pressure impingement of the membrane are arranged on the membrane, both measurement elements are arranged distanced differently far from the edge of the membrane, and the output signals of the first and the second measurement element are evaluated together in a manner such that the two measurement elements detect a differential pressure acting on the membrane, and thereby compensate the influence of the system pressure acting on both sides of the membrane.
U.S. Pat. No. 5,812,047 discloses a piezo-resistive pressure sensor comprising one type of resistors consisting of two layers, a low-doped layer and a high doped connection layer, arranged in a radial (current flows in a direction in parallel with the radial stress) and tangential (current flows in a direction orthogonal to the radial stress) geometries and having the same length and width.
The prior art also includes the following publications: EP 1785711; US 2004/045359; U.S. Pat. Nos. 5,756,899; 5,549,006; 5,432,372; 5,191,798; 4,777,826; 5,165,283; JP 59217374; JP 56043771; U.S. Pat. No. 5,243,319; US 2008/011087; U.S. Pat. No. 4,476,726; US 2008/276726; U.S. Pat. No. 5,770,883; JP 2001272293; JP 2001304998; DE 3508610; DE 3425147; U.S. Pat. No. 3,646,435; EP 0527400; U.S. Pat. Nos. 5,081,437; 5,197,334; 4,628,296; JP 6213744; EP 1087219; U.S. Pat. Nos. 6,234,027; 4,672,411; EP 0053337; U.S. Pat. No. 4,141,253; JP 53022385; U.S. Pat. No. 6,718,830; US 2010/257938; US 2007/152679; US 2003/087481; U.S. Pat. Nos. 5,303,594; 4,766,655; JP 56040735; EP 1530028; EP 1003217; U.S. Pat. Nos. 5,677,493; 5,537,882; WO 90/01153; JP 2002039891; JP 2002039886; US 2004/183150; DE 19957556; US 2011/203381; US 2002/086460; U.S. Pat. Nos. 6,510,742; 4,788,521; 5,259,248.
In the development of a 4H—SiC high temperature pressure transducer, one could not find any useful information on “piezoresistive property” of 4H—SiC, which is necessary to design piezoresistors. Indeed, there were many publications on 6H—SiC, but not on 4H—SiC. Hence, the property of 4H—SiC were experimentally tested and quite interesting features were developed. Based on the findings, the idea as to how the 4H—SiC piezoresistors should be designed and configured for a pressure transducer was reached.